1. Field of the Invention
The present invention relates generally to the field of optical measuring devices. More particularly, the present invention pertains to a photoelastic stress sensor having directional measurement capabilities.
2. Discussion of the Related Art
True directional stress measurement capability is not achieved with any present commercially available sensor technology. Within this specification, the term "stress" is defined as the force acting on an area in a solid and includes pressure type stresses and dynamic stress waves in materials. Stress as used herein also includes both short or long duration phenomena.
Stress is one of the most common parameters measured in a wide variety of fields. Stress measurements are used to assess the strength of materials, direction of applied forces, and duration of applied forces, to cite just a few of the many applications. In explosive testing, for example, stress measurement transducers are used to monitor shock wave propagation in soils, rock, and fluids. Stress measurements are utilized to characterize explosives and to quantify the environment to which selected targets are exposed. Stress measurements may be also used to characterize the thermal (through X-ray deposition) induced shock waves in new materials under development, such as composite materials, metal alloys, and ceramics.
Conventional stress measurement transducers incorporate piezoelectric or piezoresistive sensing elements. Transducers capable of monitoring explosion induced stress waves and pressures have been developed based on the piezoelectric effect in materials such as quartz and PVF (polyvinyl fluoride), and the piezoresistive effect in materials such as carbon and ytterbium.
However, conventional stress sensors, regardless of the application in which they are used, are sensitive to stress applied only in a single direction. This imposes a strict requirement to accurately position the attitude of the stress measurement transducer prior to a measurement. If the transducer is not properly oriented with respect to the direction of the applied stress, then the transducer may not accurately measure the magnitude of the applied stress. Furthermore, since the direction of the applied stress must be known in advance of a measurement, these prior art sensors cannot be used on moving devices, such as robots, to sense acoustic pressures impinging on the moving device or to measure stresses caused by another moving device.
Additional difficulties with piezoelectric and piezoresistive stress measurement transducers occur because these sensors are electronic in nature. These types of sensors are sensitive to the adverse effects of electromagnetic interference (EMI) and electromagnetic pulse (EMP) which can cause errors in the sensed stress magnitude.